The formation and development of latent images on the surface of photoconductive materials using liquid toner, the LEP process, is well known. The basic process involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resultant latent image by depositing on the image, having a background portion at one potential and a “print” portion at another potential, a finely divided electroscopic material known in the art as “toner”. The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electroscopic image. This image may then be transferred to a support surface such as paper. The transferred image may then be permanently affixed to the support surface by the application of heat, solvent, overcoating treatment or other affixing processes.
The LEP process typically utilizes a liquid developer comprising a carrier liquid having a high electric resistance and low dielectric constant and toner particles dispersed in the carrier liquid. The toner particles usually contain various components such as a binder (resin), a charge adjuvant and pigment. One problem with the LEP process as it is currently performed is that a small percentage of the pigment particles do not bind to the toner particles during the mixing process. As a result, these loose particles may not deposit properly during the printing process. These loose pigment particles cause image degradation by depositing on background areas of the photoreceptor and the final substrate. This phenomenon is known as background development. In addition to image degradation, background development has been found to shorten the life span of the photoconductor and the printing blanket.
In powder xerography background development can be reduced by using large carrier beads to scavenge stray ink particles. However, these large carrier beads typically have a size of 100-300 microns and cannot be applied in regular LEP liquid developer (LEP particle size is typically in the range of 1-10 microns). Moreover this technique requires a two component toner system whereas LEP toner is only generally comprised of one component.